Purification of lactic acid



Patented Sept. 3, 1935 romrros'rron or uic'rrc ACID Alexander Douglas Macallur'n, Niagara" Falls,

N. Y., assignorto E.

I. du Pont de Nemonrs &

Company, Inc., Wilmington, Del., a corporation of Delaware No Drawing. Applic ation January 30, 1933,

Serial No. 654,250

2 Claims.

This invention relates to the purification of lactic acid, and more particularly to anew and advantageous method of distilling lactic acid solutions.

The purification of lactic acid by distillation is difiicult because of its tendency to decompose at elevated temperatures. Heretofore, a number of methods for purifying lactic acid solutions by distillation have been proposed. Such proposed methods have included distillation in vacuo, distillation at atmospheric pressure in the presence of superheated steam at elevated temperatures, e. g., 230 to 240 C., and passing dry air through an aqueous solution at lower temperatures, e. g., 120 to 130 C., and recovering a purified lactic acid solution by condensing the ct? gases. These proposed processes have not met with success in commercial practice because they do not entirely avoid decomposition of the lactic acid and are not capable of producing the purified product in high yields. For example, ordinary vacuum distillation and other low temperature distillation methods heretofore proposed, while reducing thermal decomposition, result in the formation of considerable amounts of lactic anhydride and polymers of lactic acid. Also, steam distillation of lactic acid solutions as heretofore practiced fails to produce a pure product, the distillate often being colored.

An object of this invention is to provide a method for purifying lactic acid capable of producing a U. S. P. grade of acid in high yields. A further object is to provide a new means for distilling lactic acid solutions, which is more efficient than methods previously proposed. Other objects will be apparent hereinafter.

I have found that impure aqueous solutions of lactic acid may be purified with a very small decomposition loss by steam distillation at pressures below one atmosphere. This process results in high yields of purified solutions which require little or no further treatment to meet the U. S. Pharmacopoeia requirements for purity.

The present invention is based on my discovery that anhydride and polymer formation, occurring in ordinary vacuum distillation of lactic acid solutions, may be prevented by passing a stream of steam or water vapor into contact with the solution being vacuum-distilled. Due to the low temperatures employed, thermal decomposition is reduced to a minimum; the net result is the production of an acid of high purity in excellent yield.

In one method of carryingout my invention, steam or water vapor having a temperature oi between 50 and 120 C. is passed througha body of lactic acid in aqueous solution, which may be heated if necessary, so that the temperature of the vapors leaving the still is at 70 to 115 C., the whole operation being carried out under an absolute pressure of preferably around 10 to millimeters of mercury. Preferably, the vapors leaving the still are passed successively through two condensers, the first of which is cooled to a temperature of around to C., and the second of which is cooled to the neighborhood of 0 C. The product obtained from the first condenser consists of a pure solution containing from 90% by weight of lactic acid; the product from the second condenser is a pure but rather dilute lactic acid solution. The product from the first condenser is colorless and of a high purity, but in some cases has a slight, disagreeable odor. 1 have discovered that this odor may be entirely removed by one of several methods. One method comprises passing a current of steam, preferably at around 100 C., through the solution for a suitable period of time, e. g. a few minutes, and thereafter treating the solution with charcoal, and filtering. Preferably, I use finely divided, activated charcoal and contact it with the solution for 15 to 20 minutes before filtering. Another method for removing the odor comprises adding a small amount of hydrogen peroxide, e. g. around 1% by weight of 30% H202 solution, to the lactic acid solution and then treating the solution with steam as described above.

The purified solution may be concentrated if necessary by vacuum evaporation to 85 to 90% by weight of lactic acid.

My invention may be further illustrated by th following example:

Example A quantity of an impure lactic acid solution made by a fermentation process and containing about 44% by weight of lactic acid, was placed in a distilling vessel heated in an oil bath. The still was connected to two condensers in series and thence to a vacuum pump. The temperature in the first condenser was maintained at about 40 C. and the temperature in the second condenser at about 0 C. Steam, having a temperature of between and 0., was passed through the solution in the still; the vapors leaving the still had a temperature of about 80 C. The pressure in the distilling flask was maintained at 18-23 mm. of mercury; at a point between the second condenser and the pump the pressure varied between 9 and i0 of mercury. Analysis of the s; PATENT? OFFICE solution obtained from the first condenser showed that 86.2% of the acid in the original solution was recovered in this condensate, the condensate containing 86% by weight of lactic acid. This product met all the requirements of the U. S. Pharmacopoeia except that it contained a trace of sugars.

The above description of my invention is given merely by way of example, and it is obvious that the process may be varied considerably without departing from the spirit and scope of my invention. For instance, the temperature and pressure of the distillation may be varied considerably with favorable results. However, I prefer to maintain a temperature of distillation not greater than 120 C. and to maintain the pressure below 50 millimeters of mercury, absolute pressure. If desired, a single condenser may be used in place of the fractional condensing system described above, or a fractionating column may be employed. As illustrated by the example, it is possible to obtain a highly concentrated solution by use of a fractional condensation.

My process is well adapted for continuous operation, for instance by simultaneously flowing steam and lactic acid solution into a heated or insulated still and condensing the issuing vapors fractionally or otherwise. The process may be operated by the so-called flash distillation method, in which no appreciable amount of liquid is allowed to accumulate in the still. For such flash distillation, it is preferable to use 3 still having a long, slightly sloping bottom, down which the solution is allowed to flow. If this type of still is used, the steam may enter either with the lactic acid solution or counter-current thereto.

My process is suitable for purifying lactic acid obtained from the hydrolysis or lacto-nitrile as well as the fermentation product mentioned in the above example.

I claim:

1. A process for the purification of a crude lactic acid solution selected from the group consisting of lactic acid solutions made by fermentation and those prepared by hydrolysis of lactonitrile comprising steam distilling said solution under a pressure of less than one atmosphere and at a temperature of 60120 C., condensing the distilled vapors and treating the condensate with steam and charcoal to remove odorous impurities.

2. A process for the purification of a crude lactic acid solution selected from the group conslsting of lactic acid solutions made by fermentation and those prepared by hydrolysis of lactonitrile comprising steam distilling said solution under a pressure of 10-25 mm. of mercury at a temperature of about 80 C., passing steam through the resulting condensate and thereafter treating said condensate with activated charcoal and filtering.

ALEXANDER DOUGLAS MACALLUM. 

